The author attributes the decease of naturalism in 19th-century art to two reasons: the developing psychology of perception and the availability of new artists' materials. Monet, Seurat, van Gogh, and Cézanne are discussed in terms of some of the scientific treatises which they quoted on occasion. The difficulty of reconciling the gamut of color and intensity in nature to the limitations of a painted canvas are also discussed. The types of pigments used by some of these artists are briefly noted and the point is made that van Gogh anticipated a loss of color in his paintings. -- AATA